(a) Field of the Invention
This invention relates to a yarn storing and feeding system adapted to collect or accumulate a given quantity of yarn supplied from a yarn source for delivery to an intermittently operating yarn or consuming device and to replenish the withdrawn yarn so as to maintain the accumulated quantity at a generally constant level, and is an improvement upon the yarn storage feeder system disclosed in U.S. Pat. No. 4,226,379.
(b) Discussion of the Prior Art
As is explained more fully in U.S. Pat. No. 4,226,379 it has been known in the textile art to interpose between an intermittently operating consuming unit, especially one involving high speed delivery of the yarn thereby and a yarn supply source, a yarn accumulating or storage system capable of creating and maintaining a body of yarn for ready and immediate delivery therefrom to the yarn consuming unit, the body of yarn being repleinished as lengths thereof are withdrawn by the latter unit. An early example of this type of yarn storage and feeding system is that disclosed in U.S. Pat. No. 3,776,480 issued to John B. Lawson on Dec. 4, 1973, the disclosure of which is incorporated by reference herein as representing the general system forming the context for the present improvement and describing significant details of construction and operation of that system which are not directly germane to the present improvement. In the Lawson storage-feeder unit, the yarn is accumulated upon a generally cylindrical structure, referred to as a "drum" although it may consist of peripherally spaced segments which define a drum-like contour. The drum is associated with a hollow yarn feeder tube of generally crank-shaped or offset configuration having one end portion disposed coaxially with the drum axis and the other end offset radially from that axis exteriorly adjacent one end of the drum periphery. Yarn is delivered through the bore of the feeder tube and by imparting relative rotation between the drum and feeder tube, preferably by rotating the tube around the stationary drum, the feeder tube functions similar to the flier in conventional textile winding machines, causing windings of the yarn to be applied in coiled form upon the drum periphery. The drum is supported in cantilevered fashion so that its end opposite the feeder tube is free for withdrawal of the accumulated coils thereover and delivery to any intermittently operating yarn feeding and/or consuming device, which can, for example, take the form of a high speed shuttleless loom. The storage drum is preferably equipped with means for distributing the yarn coils applied thereto by the feeder tube uniformly or evenly axially along its length and to maintain this uniform distribution as lengths of yarn are withdrawn from its free end and fresh coils are applied to its opposite end by the feeder tube. The details of the mechanism for accomplishing the advance of the coils along the drum length are irrelevant here and fully disclosed in the cited Lawson patent and elsewhere.
To insure that the accumulated quantity of yarn on the storage drum is held at a generally constant level and to avoid overloading of the storage device, a detector or monitor is included which is actuated when the accumulated supply of yarn exceeds a predetermined maximum and upon actuation, interrupts the rotation of the flier or drum, as the case may be, to retard the rate of re-accumulation until an operating balance is restored. An insufficiency of yarn in the accumulated supply is avoided, on the other hand, by empirical measures, the rotational output of the motor driving the storage unit being adjusted to generally correspond with the previously determined average rate of consumption of the yarn by the consuming unit, and preferably at a rate slightly in excess of the average rate of consumption so that its basic operating tendency is to provide a slight excess of yarn over the quantity predicted to be needed, the monitoring means functioning to control that excess within tolerable limits.
In general Lawson type feeders are supported with their axes extending horizontally and while the driving force for rotating the feeder tube or drum member might conceivably be supplied by a flexible belt, as in conventional textile winding machines, each individual consuming unit will normally be equipped with its own storage system, and it is thus preferable to drive each storage system by a single electric motor. As is known in the art, the speed regulation of AC electric motors is relatively complex and cumbersome and a DC electric motor is more desirably employed for powering the yarn storage system. As mentioned above, the storage drum must be supported in cantilevered fashion with its supported end coupled to the driving motor. The electric motor is advantageously directly coupled to the rotatable yarn feeder tube and is mounted in coaxial relationship thereto, the yarn feeding tube penetrating centrally through the motor for axial delivery of the yarn and rotating bodily with the motor armature.
Inasmuch as space available at the consuming unit is necessarily at a premium, the axial dimension of the electric motor should be held to a minimum. A DC motor constructed with minimum axial dimension is known in the art, and is generally referred to as a flat armature or "pancake" DC motor. In this type of motor, the armature actually has the form of a thin flat "pancake-like" disc disposed in proximity to one face of an annular stator magnet assembly of only about one inch in axial thickness so that the entire axial dimension of the motor only slightly exceeds one inch. This motor is available in power ratings adequate to drive the yarn storage system in question and provides a desirable combination of characteristics needed for this purpose.
As is explained more fully in the -379 patent, the satisfactory delivery of yarn in the storage feeder system in question presents formidable difficulties in precise working control due to the essentially opposite requirements at the two ends of the system. On the one hand, the consuming unit inherently requires the delivery of yarn thereto on an intermittent or discontinuous basis, lacking any way of accommodating any appreciable excess of yarn over its actual consumption over a given period; on the other hand, the yarn supply source needs to have the yarn removed therefrom on a virtually continuous basis if effective yarn flow is to be maintained. That is to say, the yarn from the supply package must be withdrawn axially or over-end therefrom, since a bodily rotating package would be impossible to control, and over-end withdrawal inherently creates a so-called yarn balloon around the supply package. If the acceleration and deceleration forces inherently imposed upon the yarn by the consuming unit are allowed to migrate along the yarn into the balloon region, the balloon is alternately exposed to forces tending to expand or collapse it, leading to balloon instability and the occurrence of problems created by the inertia of the yarn under conditions of tension instability. Naturally, the yarn once accelerated develops inertia and tends to continue flowing if deceleration is experienced, leading to the sloughing of coils from the package as well as the creation of kinks and tangles which can result in yarn breakage or be carried forward as defects into the fabric or other product being assembled from the yarn. Similarly, if the yarn after deceleration is subjected to acceleration, the inertial resistance produces increased tension and susceptibility to yarn breakage.
In accordance with the improvement of the -379 patent, the output speed of the driving motor, and thus of the storage unit itself, is continuously monitored by a tachometer of the like, preferably in the form of a Hall effect switch sensing the rotational speed of the motor and feed tube by means of magnets equally spaced around the periphery of a rotor rotating bodily with the motor, and the current supplied to the DC motor is controlled so as to vary the actual output motor speed as necessary to correspond with a reference speed preselected, as mentioned above, to slightly exceed the average consumption rate of the yarn consuming unit. A motor speed control device found well suited for this purpose is a silicon controlled rectifier phase control unit, hereinafter referred to as the SCR unit. In order to power the DC motor, as AC line voltage, reduced if need be to an appropriate level, is rectified so as to give a full wave rectified output current consisting of a series of continuous unipolar pulses rising and falling between zero and operative voltage. The SCR unit is synchronized with the flow of pulses in the full wave rectified AC current and is actuated at the same frequency of such pulsed current but with a variable pulse period so that the motor circuit is operatively connected to the motor for only a portion or fraction of the length of each pulse, the extent of the fraction being adjusted by the SCR unit as necessary to supply the motor with an aggregate current sufficient to maintain the preset output speed. Stated in other terms, the SCR unit in effect momentarily disconnects the motor circuit for a complementary fraction of each current pulse so that the net current received by the motor increases or decreases to maintain the preselected actual motor output speed under varying driving conditions. The monitor associated with the storage member to detect the collection thereon of excess yarn above the desired predetermined yarn accumulation level is also connected to the SCR unit so as to superimpose upon the normal cycling action of that unit a further control function which temporarily deactivates the SCR unit to interrupt current flow to the motor as needed to reduce the accumulation of yarn on the storage member until the excess condition has been alleviated.
It has been discovered that the combination of the SCR phase control unit for regulating the actual motor speed with a low inductance DC motor such as the preferred flat armature DC motor results in a substantial reduction in the operating efficiency of the motor, especially in terms of output torque. In interrupting each current pulse for a fraction of its normal period, the effect of the SCR unit is to develop in the motor circuit an electric current characterized by a high RMS current in comparison to average current. As is known, the generation of heat by a motor corresponds with the RMS current it receives, while the development of output torque corresponds to the average motor current. Thus, where the RMS current is high in relation to the average current, the output torque of the motor is relatively low at a given current and an increase in motor current to increase output torque leads inherently to increased heating. By reason of its design, the low inductance armature DC motor is subjected to rather stringent maximum operating temperature which, therefore, limits the maximum current that can be applied to the motor and thus the maximum achievable output torque.